Recent advances in communication network technology are driving demand for remote devices that are coupled to a communication network. This drives a need to power these remote devices. One method of providing power to a remote device is over a hardwired communication network connection. Typical remote devices are internet protocol phones and wireless access points.
FIG. 1 shows a circuit for power transfer over a typical communication system connection to a powered device 106 that is remotely located relative to power-supplying equipment 100. The power-supplying equipment 100 provides direct current (DC) over a first pair of communication conductors 102 and a second pair of communication conductors 104 to a powered device 106. The first pair of communication conductors 102 and the second pair of communication conductors 104 typically are a twisted pair conductor, for example, those twisted pair conductors found in an Ethernet cable. The power-supplying equipment 100 has a first transformer 108 and a second transformer 110 that provide for transport of communication signals to the first pair of communication conductors 102 and the second pair of communication conductors 104 from a power-supplying equipment transceiver 112. The first transformer 108 has a first center tap 114. The second transformer 110 has a second center tap 116. The first center tap 114 and the second center tap 116 are connected to a DC power supply 118 within the power-supplying equipment 100 on the sides of the first transformer 108 and the second transformer 110 that are connected respectively to the first pair of communication conductors 102 and the second pair of communication conductors 104.
The powered device 106 has a third transformer 120 and a fourth transformer 122 that connect a powered device transceiver 124 to the first pair of communication conductors 102 and the second pair of communication conductors 104. The third transformer 120 has a third center tap 126. The fourth transformer 122 has a fourth center tap 128. The powered device 106 receives DC power from the third center tap 126 and the fourth center tap 128 over the first pair of conductors 102 and the second pair of conductors 104.
During ideal operation, a direct current (IDC) 130 flows from the DC power supply 118 through the first center tap 114, divides into a first current (I1) 132 and a second current (I2) 134 carried on the first pair of communication conductors 102. The first current (I1) 132 and the second current (I2) 134 then recombine at the third center tap 126 to reform the direct current (IDC) 130 to power the powered device 106. On return, the direct current (IDC) 130 flows from the powered device 106 through the fourth center tap 128, divides between two conductors in the second pair of communication conductors 104, recombines at the second center tap 116, and returns to the DC power supply 118. While power is being supplied, a first communication signal 136 and/or a second communication signal 138 are simultaneously carried via the first pair of communication conductors 102 and/or the second pair of communication conductors 104.
FIG. 2 shows the problems with this system on the direct current (IDC) 130 supply path. Though only the current supply is shown in FIG. 2, the direct current (IDC) 130 return path suffers the same problems. The first current (I1) 132 and the second current (I2) 134 flow in opposite directions into the third transformer 120. Ideally, no magnetic flux imbalance 200 is created in the third transformer 120 by the first current (I1) 132 and the second current (I2) 134 because the first current (I1) 132 equals the second current (I2) and because the first current (I1) 132 and the second current (I2) 134 flow in opposite directions. However, the third transformer 120 and the first pair of communication conductors 102 are not ideal due to problems such as differences in resistance of connector contacts, unequal conductor lengths, unequal conductor resistances, imperfections in windings of the third transformer 120, and other manufacturing imperfections. Thus, the resistances encountered by the first current (I1) 132 and the second current (I2) 134 are not equal. Therefore, the first current (I1) 132 and the second current (I2) 134 are not equal in magnitude when they flow through the third transformer 120.
The inequality of the first current (I1) 132 and the second current (I2) 134 creates the magnetic flux imbalance 200 in the third transformer 120 because a first magnetic flux created by the first current (I1) 132 does not completely cancel a second magnetic flux created by the second current (I2) 134. When the first current (I1) 132 and the second current (I2) 134 are small, a strength of the magnetic flux imbalance 200 is low relative to a strength of a magnetic flux created by the first communication signal 136. However, as power supplied to the powered device 106 increases, the difference between the magnitudes of the first current (I1) 132 and the second current (I2) 134 increases. Thus, the strength of the magnetic flux imbalance 200 also goes up. If power supplied increases sufficiently, the magnetic flux imbalance 200 increases in strength until the magnetic flux imbalance 200 alone saturates the third transformer 120. The magnetic flux imbalance 200 reduces a signal-to-noise ratio of the communication system connection. Saturation of the third transformer 120 causes loss of the first communication signal 136. Saturation also limits power that can be transferred between the power-supplying equipment 100 and the powered equipment 106 while maintaining ability of the communication system connection to transport the first communication signal 136.
It is cost-prohibitive to design the transformer components with sufficient tolerances to remove the flux imbalance. Accordingly, what is needed is a Power-over-Ethernet configuration that reduces or eliminates the magnetic flux imbalance.